Liquid Crystal Displays (LCDs) are widely used as information displays for notebook computers, personal computers, TVs, or the like. Their characteristics are being improved every year due to the expansion of demand. A liquid crystal panel of an LCD needs a backlight unit due to its structure that does not emit light. The backlight unit includes a variety of optical elements. In addition, the backlight unit uses optical sheets that are arranged periodically to improve luminance.
FIG. 1 shows the structure of a typical LCD. As shown in FIG. 1, the LCD 101 includes a backlight unit 20 and a liquid crystal panel 30. The backlight unit 20 includes a light source 21, a light guide plate 22, a diffuser sheet 24, and prism sheets 10. Light generated from the light source 21 is incident directly on the eye after it has been scattered through the light guide plate 22, so the light reflects the pattern of the light guide plate 22 as it is. This pattern can be appreciated clearly even after the liquid crystal panel 30 is mounted, the diffuser sheet 24 is used to remove or minimize the pattern. However, the luminance of the light is lowered sharply due to scattering in both the horizontal and vertical directions when it has passed through the diffuser sheet 24. In response, the prism sheets 10 condense the light again in order to raise the luminance. The prism sheets 10 have microscopic pitches in the form of waves. In general, a set of prism sheets including two prism sheets, the pitches of which are aligned orthogonally, are used. After the light has passed through the prism sheets, it is directed forward with a focused viewing angle, and also the luminance is improved. In the LCD to which this structure is applied, the periodic structure of the liquid crystal panel 30 and the periodic structure of the prism sheets 10 are combined with each other, thereby creating moiré fringes having a different period.
As shown in FIG. 2, the moiré fringes, which are observed when two periodic grids Gd and Gr having a pitch P are met at an angle θ, have a period D, which is determined as in Formula 1:D=P/2 sin(θ/2)   Formula 1
As seen from Formula 1, when the included angle θ defined by two periodic grids is smaller, the period of the moiré fringes creates a greater grid. The moiré fringes produce periodic light and dark patterns on the liquid crystal panel 30, thereby causing a factor attributable to degradation in the quality of the LCD 101.
In order to remove the moiré fringes in LCDs, a variety of approaches is underway. As a representative example, U.S. Pat. No. 5,280,371 discloses a technique to remove the moiré fringes. As shown in FIG. 3, in the LCD, the prism sheet 10 is tilted at a small angle with respect to the liquid crystal panel 30 (i.e., is rotated with respect to the vertical axis of the liquid crystal panel 30). This can sufficiently increase the period of the moiré fringes so that the moiré fringes do not appear on the display panel. In this LCD, the moiré fringes are reduced since the prism sheet having a periodic arrangement was rotated to tilt the arrangement. However, there is a drawback in that the luminance is lowered since the arrangement of the prism sheet is tilted.